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Graphite or Boron Nitride? Choosing the Thermal Interface for EV Charger Power Modules
The thermal-interface decision in a DC fast charger starts with an electrical question, not a thermal one: must the interface isolate the power device from the heat sink? If the joint is grounded or non-isolated, a flexible graphite sheet moves the most heat at a thin, flat bond-line. If the device must be electrically isolated, a boron-nitride filled silicone pad conducts heat while insulating, and it conforms to fill a larger or less flat gap.
Both are characterized per ASTM D5470, and both routinely run warmer in the application than the datasheet suggests, for reasons that are mechanical rather than mysterious. This guide separates the two families, explains the datasheet-versus-reality gap, and ends with an interface selector.
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The application problem: one heat path, two different interfaces
A high-power charging stage stacks power semiconductor modules against heat sinks and cold plates, and every junction in that stack is a thermal interface that either helps or hurts the junction temperature. Two interface situations recur:
- The grounded, non-isolated joint: the module base or bracket can share electrical potential with the sink. Here the interface is free to be electrically conductive, and the goal is simply the lowest thermal impedance at a thin, flat bond-line.
- The isolated joint: the device must be thermally connected to but electrically isolated from the heat sink. Here the interface must carry heat and withstand voltage at once, and the breakdown behavior matters as much as the conductivity.
The two situations point at two converted families, and swapping them costs either the thermal budget or the isolation.
Why the choice is challenging: the datasheet measures an ideal joint
ASTM D5470 is the standard method for measuring the thermal transmission of an interface material: it reports thermal impedance and conductivity under defined, controlled conditions. Real interfaces often perform worse than the headline number for three mechanical reasons: the applied clamping pressure is lower than the test pressure, so the material conforms less; the mating surfaces are not perfectly flat, so air gaps remain; and the bond-line is thicker than ideal.
The honest way to use the data sheet is as a comparison between materials and a starting point, then to confirm the interface against your real pressure, surface finish and bond-line, because the junction temperature that matters is a property of the assembled module.
Recommended material families
Both families below are characterized per ASTM D5470; grade-level impedance, conductivity and breakdown values are confirmed against the manufacturer's current technical data sheet before specifying.
For grounded, high-flux joints: flexible graphite
Graphite thermal interface material carries a lot of heat at low thermal impedance, with high in-plane and through-plane conductivity, which makes it the choice for cooling a power module at a grounded, flat, thin interface. It is a thin, conformable sheet that performs best at a flat, thin bond-line under clamping pressure, die-cut to the module footprint. Because graphite is electrically conductive, it suits the non-isolated joint only.
For isolated joints and imperfect gaps: boron-nitride silicone pads
Boron-nitride filled silicone thermal pads conduct heat while staying electrically insulating, and the soft, conformable pad fills a larger or less flat gap than a thin graphite sheet can, under modest clamping pressure. They are the choice where the power device must be isolated from the sink, and the breakdown voltage on the grade data sheet is part of the spec, not a footnote.
The supporting cast on the same charger
The thermal stack rarely travels alone. The same enclosure typically carries a conductive EMI silicone gasket (die-cut as the 2368 conductive silicone (SSP502 family)) where a seam must shield under CISPR 11, an EPDM closed-cell foam seal on the outdoor cabinet joints, and an ePTFE pressure-equalization vent so the sealed cabinet does not pump moisture as it heats and cools. Those parts are covered in the companion enclosure guide; they matter here because the TIM, the gaskets and the vent are usually quoted and converted as one kit.
Isolation, Gap and Flux: TIM Direction Selector
At a grounded, flat, thin, clamped interface, flexible graphite moves the most heat at low thermal impedance per ASTM D5470; confirm the grade's impedance at your pressure and bond-line on the data sheet.
Illustrative only; confirm against current material data and a drawing review. This selector returns a material family direction, not a grade or a guaranteed thermal result. Thermal impedance, conductivity and breakdown voltage are confirmed on the manufacturer's current technical data sheet at your clamping pressure, flatness and bond-line, and the junction temperature belongs to the assembled module.
Design and converting considerations
- Send the interface, not just the wattage. The heat to move, the gap, the flatness and the achievable clamping pressure together decide the family and gauge; the wattage alone does not.
- Treat breakdown voltage as a spec line. For isolated joints, the boron-nitride pad's breakdown rating on the grade data sheet is part of the electrical design, confirmed against the working voltage with margin.
- Mind the bond-line both ways. Too thick wastes thermal budget; too thin risks voids on an uneven surface. The pad or sheet thickness is matched to the measured gap and flatness.
- Die-cut to the module, not the sheet. Cutouts for posts and fasteners, tabs for placement, and kiss-cut liners make the interface repeatable on the line; H-O converts the footprint as drawn.
- Quote the kit together. TIM, EMI gasket, enclosure seal and vent share the same drawing review and ship as one kitted set when the program wants it.
Common failure modes
| Failure mode | Root cause | Consequence |
|---|---|---|
| Interface runs warmer than the datasheet | Lower pressure, less flat surfaces, thicker bond-line than the ASTM D5470 test condition | Junction temperature above budget; confirm at real pressure, flatness and bond-line |
| Isolation lost at the joint | Conductive graphite specified where the device needed isolation | An electrical fault path; the boron-nitride pad was the call |
| Heat budget given away | A conformable isolating pad at a grounded, flat joint that graphite could serve | Higher impedance than necessary; ask the isolation question first |
| Voids at an uneven interface | A thin sheet asked to fill a gap it cannot conform to | Air gaps in the heat path; a conformable pad fills the rough joint |
| "Certified TIM" requested | Charger standards applied to a raw material | IEC 61851 applies to the charging system and equipment; the material supports the tested charger |
How H-O Products helps
H-O Products is a precision die-cut converter serving EV and battery programs. We convert graphite thermal materials, boron-nitride silicone pads, conductive silicone, EPDM foam and ePTFE membrane into the thermal, EMI, sealing and venting parts a charger cabinet needs, cut to your module footprints and enclosure geometry. As an ISO 9001:2015 certified organization, we help you answer the isolation question, match the family and grade direction to your gap, pressure and heat, and convert samples for thermal verification.
We do not produce the raw stock; impedance, conductivity and breakdown values are confirmed against the manufacturer's current technical data sheet, and the junction temperature and charger compliance belong to your tested equipment.
Working a charger thermal budget?
Tell us the heat to move, the gap and flatness, the achievable pressure, and whether the interface must isolate. We will recommend a family and grade direction and convert samples on your footprint.
Samples typically ship in 3 to 5 business days for common die-cut configurations; standard production runs about two weeks; special orders are quoted to the job.
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Related reading
Frequently asked questions
How do I choose between a graphite and a boron-nitride thermal interface for a power module?
Start with whether the interface must electrically isolate the device. Graphite has very high conductivity at low thermal impedance but is electrically conductive, so it suits a grounded or non-isolated joint with a thin, flat, clamped bond-line. A boron-nitride filled silicone pad is electrically insulating and conformable, so it is the choice where the device must be isolated from the heat sink or where the gap is larger or less flat than a thin sheet can fill. Both are characterized per ASTM D5470.
Why does my real thermal interface run warmer than the datasheet value?
ASTM D5470 measures thermal impedance and conductivity under defined, ideal conditions. Real interfaces often perform worse for three reasons: the applied clamping pressure is lower than the test pressure so the material conforms less, the mating surfaces are not perfectly flat so air gaps remain, and the bond-line is thicker than ideal. Use the datasheet to compare materials and as a starting point, then confirm against your real pressure, surface finish and bond-line.
Can a thermal interface material be certified to a charger standard like IEC 61851?
No. IEC 61851 applies to the charging system and equipment, not to a raw material. A thermal interface material supports a charger that is built and tested to the standard, the same way ASTM D5470 characterizes the TIM, CISPR 11 measures the enclosure's emissions, and IEC 60529 classifies the enclosure's ingress protection: each result is a property of the tested assembly under its real conditions.
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Last updated · Reviewed by H-O Products engineering (ISO 9001:2015 certified converter, Winsted, CT).
References
- ASTM D5470, Standard Test Method for Thermal Transmission Properties of Thermally Conductive Electrical Insulation Materials. ASTM International.
- IEC 61851, Electric vehicle conductive charging system. International Electrotechnical Commission.
- CISPR 11, Industrial, scientific and medical equipment: radio-frequency disturbance characteristics, limits and methods of measurement. CISPR / IEC.
- Manufacturer technical data sheets for the graphite and boron-nitride grades discussed; values are grade-specific.
This article cites general engineering principles and published standards (ASTM D5470 for thermal-interface characterization, IEC 61851 for EV charging systems, CISPR 11 for industrial-equipment emissions). Material names refer to material families H-O Products converts. Thermal impedance, conductivity and breakdown-voltage values are grade-specific and depend on clamping pressure, flatness and bond-line; confirm all values against the manufacturer's current technical data sheet and an engineering drawing review.
Junction temperature and charger compliance belong to the tested equipment. All parts are custom manufactured to order and minimum production run quantities apply. H-O is a die-cut converter, not a raw-material producer, and does not certify the finished assembly.